This application is related to and claims priority from Japanese Patent Application No. 11-227211, filed Aug. 11, 1999, the entire disclosure of which is incorporated herein by reference.
The present invention relates to large scale integrated circuit chips (herein sometimes termed LSI chips) having semiconductor elements that provide high speed, high integration, high heat-generation density, high power dissipation, and large size. In particular, the invention relates to a cooling technique for removing, with low thermal resistance, the heat generated by a multi-chip module (MCM) in which a large number of LSI chips are mounted at high density.
As computers such as large mainframe and server computers, and super computers, gain higher computing speed and larger storage or memory capacity, LSI chips for use in them have become higher speed, highly integrated, larger and resulting in much greater heat-generation. To transmit signals at high speed among the LSI chips, the electric wiring among them should them as short as possible. Accordingly, such computers now often include MCM""s each having a number of LSI chips mounted densely on a multi-layer circuit substrate. To avoid delay in signal transmission among the LSI chips, it is important for the MCM to be provided with suitable cooling to improve LSI chip operation by reducing the temperature and equalizing the distribution of temperature among the chips.
One conventional cooling device for removing heat generated by LSI chips mounted in an MCM includes a water-cooling jacket over the top of the MCM. Cooling water at a predetermined temperature flows in the jacket. For example, Japanese patent laid-open publication No. 08-279578 discloses a cooling device for an MCM. This cooling device includes a water cooling jacket having cooling channels formed in it. Cooling fins extend in parallel in the cooling channels. The cooling channels terminate in headers for reversing the direction of flow of the cooling water. The cooling water flowing in the jacket flows successively through the cooling channels, reversing in the direction of flow, to remove heat generated by the LSI chips in the MCM.
A common method for improving the cooling performance of a water cooling jacket involves deepening and/or narrowing the cooling channels in the jacket. For example, the deepened cooling channels make it possible to increase the heating surface areas, and to improve the cooling performance. If the same pump is used for cooling-water circulation, however, the cooling performance worsens because the total sectional area of the cooling channels increases, reducing the average flow velocity of the cooling fluid and the fin efficiency, as the channels deepen, although the decrease in pressure loss of the cooling fluid increases the total circulating flow rate of the cooling water. Accordingly, both of the foregoing actions or effects restrict the improvement of the cooling performance of a water cooling jacket.
If the total circulating flow rate is constant, the average flow velocity of the cooling fluid increases, thereby improving the cooling performance, but the pressure loss of the fluid increases quickly or suddenly, as the cooling channels narrow. If the same pump is used for cooling-water circulation, the increase in pressure loss reduces the total circulating flow rate, thereby restricting the improvement of the cooling performance of a water cooling jacket.
As the cooling channels narrow, the fluid flow through them transits or changes to laminar flow. This can be explained with a Reynolds number, a characteristic number representing the flow condition through cooling channels. The Reynolds number is found by multiplying the hydraulic equivalent diameter of cooling channels by the average flow velocity of the cooling fluid flowing through the channels, and dividing the resulting product by the kinematic viscosity of the fluid. The flow of cooling fluid through cooling channels is said to be turbulent if the Reynolds number is larger than a range between about 2,300 and about 3,000, and to be laminar if the Reynolds number is smaller than this range. As the cooling channels narrow, their hydraulic equivalent diameter decreases nearly in proportion to the channel width. On the other hand, the average flow velocity of the cooling fluid flowing through the cooling channels increases in inverse proportion to the channel width, but the total circulating flow rate decreases, and consequently the increase in the average flow velocity of the fluid is small. As a result, the Reynolds number decreases monotonically. Accordingly, as the cooling channels narrow, the flow of the cooling fluid changes to laminar flow. The foregoing description has been made on the assumption that the number of cooling channels is constant. If the cooling channels are narrower and the fins are thinner, however, the number of cooling channels is larger and accordingly the total sectional area of the channels is larger. This reduces the average flow velocity of the cooling fluid, making the Reynolds number even smaller. It is known that, in general, as the flow through cooling channels changes from turbulent flow to laminar flow, the Reynolds number decreases and the heat transfer performance of the channels decreases. Therefore, the improvement in cooling performance of a water cooling jacket is limited if its cooling channels are merely deepened and/or narrowed.
To remedy the foregoing disadvantages of methods for improving the cooling performance of general or common water cooling jackets, Japanese patent laid-open publication No. 7-70852 discloses prior art for improving the cooling performance of a water cooling jacket by locally reducing the sectional area of the cooling channel without changing the size of the entire channel. FIG. 7 is a cross section showing the principle of the improvement in cooling performance of a conventional water cooling jacket.
With reference to FIG. 7, a cooling device includes a cooling plate 41 having a flow channel 44. The cooling plate 41 is positioned in contact with an electronic part 42 at a place 43. An inner wall of the flow channel 44 has a protrusion 46 formed over the place 43. The protrusion 46 locally accelerates the flow of the cooling fluid through the flow channel 44, improving the heat transfer performance of that portion of the channel wall which is contacted by the accelerated flow. Immediately in front of and behind the protrusion 46, however, a separating phenomenon occurs in the fluid flow, decreasing the heat transfer performance. This phenomenon is described in Yoshiro Kato""s xe2x80x9cDennetsugaku Tokuronxe2x80x9d published by Yokendo on Oct. 5, 1984, page 211. Thus, the protrusion 46 and the electronic part 42 are in the same position on inner or outer both sides of an inner wall 45 of the cooling plate 41. If the protrusion 46 is smaller than the area of the place 43 where the electronic part 42 is in contact with the wall 45, the average cooling performance of the contact place 43 is such that the decrease in heat transfer performance in front of and behind the protrusion 46 and the increase in heat transfer performance just under the protrusion 46 cancel each other. This restricts the improvement in cooling performance of the cooling plate 41. If the protrusion 46 is larger than the area of the contact place 43, the region where the cooling fluid accelerates is longer, increasing the pressure loss of the fluid. Consequently, in either case, the improvement in cooling performance of the cooling plate 41 is limited.
Japanese patent laid-open publication No. 2-257664 discloses a device for cooling an MCM, which includes a large number of LSI chips mounted on a multi-layer circuit substrate. This cooling device includes a one-piece or integral water cooling jacket, which has cooling micro channels formed therein. The back sides of the LSI chips are soldered to the back side of the water cooling jacket. The cooling water flowing in the jacket removes the heat generated by the chips. The water cooling jacket serves as a cap for hermetically sealing the MCM.
If the one-piece water cooling jacket of this cooling device is larger in size for higher cooling performance, its heat capacity is even larger. In other words, because the back side of the one-piece jacket is soldered to the back sides of the LSI chips, it is necessary to melt or solidify the solder on the chips when assembling or disassembling the cooling device. When the solder is melted or solidified, a large temperature excursion is produced in the cooling jacket itself and/or between the jacket and other parts. This lowers the reliability of the LSI chip interconnections and the hermetic seal reliability of the MCM.
In recent years, as there has been need for the computing speed of computers such as large-scale or super computers to be even higher, LSI chips for use in them have been even more integrated, higher in heat generation density, larger in size, higher in power dissipation, and more densely mounted. For example, the LSI chip size may be as large as 15-20 mm square, and the heat generation density of the LSI chips may be as high as 100-150 W/cm2. Very important problems arise with devices for cooling MCM""s having numbers of LSI chips mounted on them which are high in heat generation density, large in size and high in power dissipation. These problems have not been serious for the conventional devices for cooling MCM""s having LSI chips which are about 10 mm square in size and/or about 10-50 W/ cm2 in heat generation density.
One of the foregoing problems is that, because LSI chips are very high in heat generation density and/or power dissipation, their temperatures vary widely even with slight changes in cooling condition, and it is therefore difficult to lower the temperature rise of the chips and equalize the temperatures of the chips in comparison with the conventional cooling devices. The reason for this is that, for high computer performance, it is important to equalize the electrical characteristics of the many LSI chips mounted in an MCM. Accordingly, it is important to provide a cooling device for stable operation of a large number of LSI chips. This may be achieved by reducing the temperature rise of the LSI chips and the temperature difference among the chips so that no delay shift occurs in signal transmission among the chips.
Another problems is that, as LSI chips become larger in size and higher in heat generation density, it is more difficult to equalize the temperature distribution in the chips. The reason is that to equalize the electrical characteristics of the circuit elements in highly integrated LSI chips, it is important to equalize the temperature distribution in the chips. To prevent decreasing the reliability of the MCM, it is important to reduce the thermal deformation produced by the nonuniformity of the temperature distributions.
Another problem is that, for larger power dissipation of MCM""s, a larger amount of cooling fluid needs to flow to cool the MCM""s. It is therefore important to improve the cooling performance without increasing the pressure loss of the cooling fluid.
A further problem is that, for larger sizes of MCM""s, the productivity of the MCM""s, such as their capability to be assembled and disassembled, is considered to be very important in comparison with the conventional cooling devices. In particular, when soldering the back side of a semiconductor device and the sealing top plate of an MCM together, or hermetically sealing an MCM with solder or the like, it is necessary to heat and cool the whole MCM to melt and solidify the solder. For the thermal deformation produced when the MCM is heated and cooled to be kept small, it is important to equalize the thermal expansion coefficient of the MCM materials and/or make the heat capacity of each member small.
The present invention provides an MCM cooling device which efficiently reduces the temperature of highly integrated LSI chips generating substantial heat, which are densely mounted. The invention provides superior productivity of MCM""s, enabling easier assembly and disassembly, and protects the LSI chips reliably.
In accordance with a first aspect of the present invention, a device is provided for cooling an MCM including a number of LSI semiconductor devices, packaged or unpackaged. The device includes a sealing top plate for removing the heat generated by the semiconductor devices. The MCM cooling device includes a plurality of parallel cooling channels formed in the sealing top plate and cross grooves extending partially across and through the cooling channels. The cooling channels are covered with a cooling channel cover, which has turbulent promoters formed on an inner wall thereof integrally with the channel cover. The cross grooves are positioned midway among adjacent semiconductor devices. By engaging with the cross grooves, the turbulent promoters disturb the flow of cooling fluid through the cooling channels to improve the cooling performance of the cooling channels. For the heat transfer enhancement of the cooling channels to be maximum locally near the middle of each semiconductor device, the height of the turbulent promoters may range between {fraction (1/10)} and {fraction (1/16)} of the pitch at which the semiconductor devices are arranged.
In accordance with a second aspect of the invention, an MCM cooling device is provided which has a plurality of parallel cooling channels formed in a cooling channel plate or a sealing top plate, cross grooves extending partially across the cooling channels, and turbulent promoters for engagement with the cross grooves. The turbulent promoters are formed on and integrally with an inner wall of a cooling channel cover. The cooling device also has a bypass channel formed in the inner wall of the cooling channel cover over the cooling channels so that cooling fluid can partially bypass said cooling channels through the bypass channel. This means both reduces the pressure loss of the cooling channels and improves their cooling performance.
In accordance with a third aspect of the present invention, an MCM cooling device is provided which has cooling channels formed in the sealing top plate of an MCM. The sealing top plate is made of a high thermal conductive ceramic having a thermal expansion coefficient matching with that of the multi-layer circuit substrate of the MCM. The cooling channels are covered with a cooling channel cover, which is a separate metallic member. The MCM includes semiconductor devices such as LSI chips or LSI packages, each of which contains an LSI chip. When the sealing top plate is soldered directly to the back sides of the semiconductor devices, the cooling channel cover is removed so that the heat capacity of the sealing top plate is small. A fastening means fastens the metallic cooling channel cover and the hermetic sealing frame of the MCM tightly to each other with an O-ring or another packing member interposed between a peripheral portion of the sealing top plate and the cooling channel cover. This prevents the leakage of cooling fluid while maintaining the strength of the ceramic sealing top plate. Consequently, the MCM cooling device raises the productivity of MCM""s, such as their capability to be assembled and disassembled.